1. Field of the Invention
The present invention relates to an apparatus and a method for processing two-tone image data so as to smooth and magnify a relevant image. The above-mentioned apparatus and method may be applied to an image processing apparatus such as a facsimile apparatus, a digital duplicator having a facsimile function, a digital duplicator not having the facsimile function, a printer such as an optical printer such as a laser printer, and so forth. Data to be processed by the apparatus or method is, for example, data such as that received through the facsimile function. The above-mentioned smoothing of a relevant image is removing jaggies or aliasing from the relevant image. The jaggies or aliasing means "stairsteps" undesirably appearing in inclined lines and curves of an image when the image is represented at a relatively low resolution. The stairsteps are a series of steps consisting of a line such as a boundary line between a zone consisting of white pixels and a zone consisting of black pixels in an image and are like steps of a stair.
2. Prior Art
A smoothing technique will now be described. The smoothing technique is used in processing given two-tone image data so as to magnify a relevant image an integer number of times. In such processing, a group of pixels located around a relevant pixel in the image are used as data to determine a manner in converting the relevant pixel into a plurality of pixels. Hereinafter, converting each pixel into a plurality of pixels will be referred to as magnifying the pixel. Further, an operation to be performed on an image such that a number of pixels (dots) constituting the image is increased will be referred to as magnifying of the image. As a result of such magnifying, it is possible to smoothly interpolate between steps of the jaggies which appear as a result of the magnifying. In this technology, output data is also two-tone image data. Such a technology has been made practicable in a facsimile apparatus in commercial use.
Another technique is for removing the jaggies applicable to a printer or the like having a capability of printing in a multiple-tone printing manner. The technique attempts to obtain high-quality images as a result of converting pixels consisting of the jaggies in an image represented by two-tone image data into pixels represented by multiple-tone image data. A typical technique of the above-described technique was made practicable as a Resolution Enhanced Technology (which will be abbreviated `RET`, hereinafter) of HP (Hewlett-Packard). Further, similar techniques have been made practicable by other printer manufacturers.
Recently, a product such as that having a block structure shown in FIG. 1 has appeared on the market.
The product is a system having a printer 1, a scanner 2, and a modem 3 so as to constitute a digital duplicator having a facsimile function. The system performs a digital duplicating function of reading in an original image through the scanner 2 and printing out the thus-read-in image through the printer 1. Latest digital duplicators have a function of printing an image in the multiple-tone printing manner as mentioned above. The digital duplicating function is an essential function of the system. Further, the system has a printer function of printing out a document produced by a word processing function of a computer 4 connected to the printer 1. The system further has a network function of printing out data obtained through a network 5 connected to the printer 1. The network function also includes a function of transferring an image to the network 5, the image having been read in through the scanner 2. The system further has a scanner function of storing images obtained through the scanner 2 in a file 6 (a recording medium such as an optical disc having a huge data storing capacity) and thus forming a data base of the thus-stored images. The system further has a facsimile function of communicating facsimile images through a public line 7 which connects the modem 3 with a facsimile machine 8.
FIG. 2 shows magnifications required in magnifying one of three different sorts of images, which have been transmitted through facsimile transmission methods, so as to print out the image through one of printers having three different resolutions, 300 dpi, 400 dpi and 600 dpi. In the leftmost column of FIG. 2, indicating resolutions of facsimile reception images, "8.times.3.85" means "8 (dots/mm).times.3.85 (lines/mm)"; "8.times.7.7" means "8 (dots/mm).times.7.7 (lines/mm)"; and "6.times.15.4" means "16 (dots/mm).times.15.4 (lines/mm)". Similarly, in FIG. 2, the printer resolutions of "300 dpi (12.times.12)","400 dpi (16.times.16)" and "600 dpi (24.times.24)" mean "300 dpi (12 (dots/mm).times.12 (lines/mm))", "400 dpi (16 (dots/mm).times.16 (lines/mm))" and "600 dpi (24 (dots/mm).times.24 (lines/mm))" respectively
How FIG. 2 should be viewed will now be described. In an example, if an image of 8.times.7.7 (second or middle row of FIG. 2) which have been transmitted through a facsimile transmission method is printed through a printer having a resolution capability of 400 dpi (second or middle column of FIG. 2), it is necessary to magnify the image twice in a horizontal direction and twice in a vertical direction that is, 2.times.2 times.
Why the magnifying is necessary will now be described. If image data of the image of 8 (dots/mm).times.7.7 (lines/mm) is used to print out the image through the printer having the resolution capability of 400 dpi (16 (dots/mm).times.16 (lines/mm)), each of a horizontal dimension and a vertical dimension of a resulting image is approximately half of a respective one of those of the original image. This is because a number of dots (8) per unit length in the horizontal direction of the original image is 1/2 of a number of dots (16) per unit length in the horizontal direction of the resulting image, each dot of the original image corresponding to a respective dot of the resulting image. Similarly, a number of lines (7.7) per unit length in the vertical direction of the original image is approximately 1/2 of a number of line (16) per unit length in the vertical direction of the resulting image, each line of the original image corresponding to a respective line of the resulting image. In order to maintain the dimensions between the original image and the resulting image, it is necessary to magnify, that is, to double the number of dots per unit length in the horizontal direction and to double the number of lines per unit length in the vertical direction of, the original image, before the printing out of the resulting image.
There is demand for system to be devised which simultaneously has a function of image magnifying such as that shown in FIG. 2 and also a function of the above-mentioned RET on the same image data. This system magnifies an image, represented by two-tone image data obtained through the facsimile method, while smoothing the image.
In order to respond to the above-mentioned demand, a certain image processing method may be contrived. The certain image processing method is a method of processing an image represented by two-tone image data and obtained through the facsimile method. The certain image processing method uses a conventional smoothing technique so that an operation of the above-mentioned smoothly interpolating method is performed together with magnifying the image an appropriate number of times. Then, an operation of the RET is performed on the magnified and smoothly interpolated image. Then, the image is printed out in the multiple-tone printing manner.
However, in order to realize the above-described image processing method, it is necessary to use two separate ASICs (Application-Specific Integrated Circuits), specifically, for example, gate arrays, one for the above-mentioned smoothly interpolating operation and the other for the above-mentioned RET operation. Therefore, high costs are required to realize the method. Further, circuit design of the ASICs and/or other peripheral circuits depend on a resolution of a printer which is provided with the ASICs. Since the resolutions of printers are at various levels, it is required that the circuit design be performed in various manners to match the resolutions of the particular printers.
Further, there may be some pixels unexpectedly left without having undergone the smoothly interpolating operation in the smoothing technique for the two-tone image data. The RET operation may not have an effect on the thus-left pixels. As a result, an image, having a part in which a printing quality is degraded, may be obtained. Why the RET operation may not have an effect on the thus-left pixels will now be described. There may be a case where a stairstep part is present in a boundary line between a zone consisting of black pixels and a zone consisting of white pixels. In the stairstep part, a number of pixels constituting each one of the vertical step dimension and the horizontal step dimension is equal to or larger than two. If an image including such a stairstep part is simply magnified by a factor of 2 in both the horizontal and vertical dimensions, the each one of the vertical step dimension and the horizontal step dimension is equal to or larger than four. A possibility of such a stairstep part, having long step dimensions, being appropriately smoothed due to the RET operation is low.